1. Field of the Invention
The present invention relates to nonreciprocal circuit devices for use in high-frequency bands including the microwave band, such as isolators and circulators, and more particularly, to nonreciprocal circuit devices which allow mobile communication units to be made compact and inexpensive when used in them.
2. Description of the Related Art
Nonreciprocal circuit devices such as lumped-constant isolators and circulators have characteristics in which attenuation is very low in the direction in which a signal is transferred and it is very high in the reverse direction. FIG. 9 shows a configuration of such an isolator.
This isolator includes a magnetic closed circuit formed mainly of an upper yoke 2 and a lower yoke 8; a permanent magnet 3; a spacer member 4; a magnetic assembly 5 formed of three central conductors 51, 52, and 53 and a ferrite 54; and a resin case 7. The port sections P1 and P2 of central conductors 51 and 52 are connected to input and output terminals 71 and 72 provided on the resin case 7 and matching capacitors Co. The port section P3 of central conductor 53 is connected to a matching capacitor Co and a terminating resistor R. An end of each capacitor Co and an end of the terminating resistor R are connected to ground terminals 73.
The spacer member 4 is disposed between the permanent magnet 3 and the magnetic assembly 5. When the upper yoke 2 fits in the lower yoke 8, the spacer member 4 presses and secures the magnetic assembly 5 and the resin case 7 to the lower yoke 8, and the matching capacitors Co and the terminating resistor R to the resin case 7. In addition, the spacer member 4 presses and secures the port sections P1 to P3 of the central conductors 51 to 53 to the matching capacitors Co, the terminating resistor R, and the input and output terminals 71 and 72, all of which are disposed in the resin case 7. In other words, the spacer member 4 fills an internal gap of the nonreciprocal circuit device and is used for stable holding and fixing of the components disposed inside the nonreciprocal circuit device, such as the magnetic assembly 5, the matching capacitors Co, and the terminating resistor R.
FIG. 10 is an equivalent circuit diagram of the isolator. As shown in FIG. 10, in the conventional isolator, the ports P1, P2, and P3, the tips of the central conductors 51, 52, and 53, are connected to the matching capacitors Co serving as matching circuits, and port P3 is connected to the terminating resistor R. Each inductance L corresponds to an equivalent inductance formed by the ferrite 54 and one of the central conductors 51, 52, and 53.
FIG. 22 shows a second conventional isolator. This isolator includes a magnetic closed circuit formed mainly of an upper yoke 2 and a lower yoke 8; a permanent magnet 3; a magnetic assembly 5 in which three central conductors 51, 52, and 53 overlappingly intersect each other on a ferrite 54; and a resin case 7. On the lower surface of the lower yoke is a terminal board 9 on which input and output electrodes 91 and 92 and a ground electrode 93 are formed. The port sections P1 and P2 of central conductors 51 and 52 are connected to input and output connection terminals 71 and 72 provided on the resin case 7 and matching capacitors Co. The port section P3 of a central conductor 53 is connected to a matching capacitor Co and a terminating resistor R. An end of each capacitor Co and an end of the terminating resistor R are connected to ground terminals 73. The input and output connection terminals 71 and 72, and the ground terminals 73 are connected to the input and output electrodes 91 and 92, and the ground electrode 93, respectively.
The terminal board 9 increases the degree of freedom in designing the signal input and output sections of the isolator by changing the shapes and positions of the signal input and output sections, as required, and assures stable and positive connection to a mounting board on which the isolator is to be mounted. The input and output electrodes 91 and 92, and the ground electrode 93 are formed as respective pairs of electrodes formed on corresponding main surfaces of the terminal board 9. Each pair of electrodes formed on both main surfaces are connected to each other by a through hole or an end-face electrode.
FIG. 23 is an equivalent circuit diagram of the isolator. As shown in FIG. 23, in the conventional isolator, the ports P1, P2, and P3, i.e. the tips of the central conductors 51, 52, and 53, are connected to the matching capacitors Co serving as matching circuits, and port P3 is connected to the terminating resistor R. Each inductance L corresponds to an equivalent inductance formed by the ferrite 54 and the central conductors 51, 52, and 53.
This isolator for use in a transmission and receiving circuit section of an antenna sharing circuit in a mobile communication unit such as a portable telephone or an automobile phone. The isolator is surface-mounted to a mounting board of which input and output transmission lines and a ground electrode are formed on the front surface and a ground electrode is formed on almost all areas of the rear surface.
An amplifier built into such a communication unit is nonlinear in general, and it causes extraneous emissions, that is, spurious signals, such as signals having frequencies which are multiples of that of the fundamental wave, especially the second harmonic and the third harmonic. Since these extraneous emissions may cause radio interference and malfunctions in the power amplification sections of other communication units, it is required to suppress the emissions to a certain level according to a standard.
An isolator also functions as a band-pass filter due to its transmission-direction characteristics. Therefore, attenuation is large even in the transmission direction in frequency bands away from the pass band. Since the main purpose of an isolator is not to obtain attenuation outside its frequency band, however, the desired attenuation cannot be obtained at the frequencies (especially those of the second harmonic and the third harmonic) of extraneous emission in the conventional isolator. Therefore, an additional filter is employed in the conventional communication unit to attenuate extraneous emissions.
In other words, when the conventional isolator is used, a filter for attenuating extraneous emissions is required as described above. The cost of components used increases by that of the filter, and the communication unit becomes large in size. The unit cannot be made compact or inexpensive.